TAKE a moment to admire—and fear—the ascent of America’s big-five tech firms. Apple, Alphabet, Microsoft, Amazon and Facebook have recently become the five most valuable listed companies in the world, in that order. With a total market value of $2.9trn, they are worth more than any five firms in history.

Elevated tech valuations used to be a sign of hysteria. Today’s investors believe they are making an ice-cold judgment that these firms are the dominant oligopolies of the 21st century and will extract a vast, rising, flow of profits. There is one gnawing doubt, however: the formidable five’s cash-rich balance-sheets, which are built as if they expect a crisis, not to dominate the world.